Quartz glasses which have substantially no impurities and which in particular are substantially anhydrous are required for the manufacture of optical waveguides as well as for the manufacture of lamp envelopes, in particular for halogen lamps or gas discharge lamps. The manufacture of such glasses via the conventional route of manufacturing from a glass melt is impeded due to impurities which are present.
For this reason, it has been proposed, for the said application purposes, to manufacture highly pure glasses by sintering porous green bodies of very fine quartz glass particles having a particle size in the range from 1 to 500 nm. Since the green bodies are porous and hence gas pervious, they may be subjected to a purification step in a heated gas atmosphere reacting with impurities present and, in the purified condition, they are then sintered to transparent glass at temperatures around 1500.degree. C. Disturbing impurities are, for example, hydroxyl ions and ions and particles of the transition metals.
Such green bodies may be formed from suspensions via a sol-gel conversion but also porous green bodies of pulverised starting materials may be formed, for example, by centrifuging followed by form stabilisation. A method of manufacturing glass bodies which are to be used as preforms for the manufacture of optical waveguides in which open-pore green bodies are purified with a chlorine-containing purification gas heated at a temperature in the range from 600.degree. to 900.degree. C. is known from German Patent Application P 35 11 439.8. This method has proved suitable in particular for removing hydroxyl ions. The essential idea of said purification process is that impurities present diffuse to the surface of the very small quartz glass particles by means of solid state diffusion, react there with the chlorine-containing atmosphere to form volatile compounds and are forced by periodic pressure gradients, through the open pores to the surface of the preform.
A problem in this method is that impurities which are present from the beginning at the surface of the highly dispersed quartz glass particles do not immediately react completely to be transported away but have sufficient time to also diffuse into the material due to the high temperatures of the purification process. This may impede a purification process essentially.
Examples of the said impurities are ions and particles of copper, vanadium, chromium, molybdenum, tungsten, manganese, iron, cobalt or nickel which result from the mechanical abrasion during mixing and extrusion of the starting material to manufacture a green body and have deposited on the surface of the powder particles taking part in the composition of the starting mass.